schema/excluded_schema can be a list

[rienafairefr]

As part of introducing multiple schema inclusion/exclusion in migra, I think this could be an addition:

The get_inspector call can now receive schema/exclude_schema as a list, keeping the backward compatibility with passing simple strings. I added multiple_schemas and exclude_multiple_schemas methods to PostgreSQL, breaking the filter_schemas signature compatibility (deep enough in schemainspect that I think breaking it is fine, but that could be adapted). I was tempted to add arguments schemas/exclude_schemas, but didn't in the end.

Added the corresponding unit tests, and AFAIK I'm not breaking the existing tests (tested locally on pg 12)

it fixes #68 by @daa